In a semiconductor production apparatus, an exhaust pump P such as the one shown in FIG. 8, for example, is used as a means for exhausting gas expelled from a processing apparatus such as an etching apparatus to the outside. This exhaust pump P has a rotating body R formed by a cylindrical part 1 and a blade part 2 and driven by a motor M to rotate around a rotor shaft 3.
Gas molecules present on an inlet port 4 side of the exhaust pump P are given a downward momentum by the rotating blade part 2 and conveyed to upstream of a threaded part 5, compressed there, and exhausted to the outside from an outlet port 6.
It is known that products deposit inside the exhaust pump such as a turbomolecular pump during such exhaustion of gas (see, for example, paragraph [0014] of Japanese Patent Application Laid-open No. 2003-232292). Products particularly tend to deposit in part S of a gas passage shown in FIG. 8.
Japanese Patent Application Laid-open No. 2003-232292 discloses a method of detecting products deposited inside a pump (referred to also as “internal deposit”). According to this detection method, motor current is detected as the motor rotates rotary blades of a turbomolecular pump, and the detected motor current is compared with a preset value. If, as a result of the comparison, the detected motor current is equal to or greater than a preset value, a notification is made that a time for maintenance has arrived (see paragraph [0022] and others in Japanese Patent Application Laid-open No. 2003-232292).
However, exhaust pumps including the turbomolecular pump of Japanese Patent Application Laid-open No. 2003-232292 are used in various processes by end users, i.e., pumps are used for a variety of gases and in various flow rates depending on the contents of processes. Therefore, the current value of the motor that rotates the rotating body of the exhaust pump variously changes in accordance with a type and flow rate of gas flowing through the exhaust pump.
With the deposit detection method mentioned above, the detection of product deposition is based on a preset current value, so that correct detection of deposits is impossible in a process to which the preset current value is not adapted.
In order to avoid erroneous detection or false alarms, the preset current value must be changed in accordance with conditions of use of the exhaust pump, which must be closely examined beforehand, such as what type of gas will be used in what flow rate in the process in which the exhaust pump is to be used, which is a time-consuming and costly task.
The applicant of the present invention has previously filed an application on a novel invention relating to detection of a deposit inside a pump, wherein an initial motor current and a current motor current are determined in an initial step and a later step, respectively, and an amount of change in the motor current relative to the initial value is determined, based on which deposition in the pump is detected, in order to correctly detect and alert to a deposit of products inside the exhaust pump used in a process, whatever process it may be, irrespective of the type or flow rate of the gas used in the process (see WO 2011/145444).
As shown herein, in the case with the exhaust pump P of FIG. 8, as products deposit in part S of the gas passage in a lower part of the cylindrical part 1, pressure in a lowermost part of the exhaust pump P below the blade part 2 increases. This increases the load on the motor M, so that the motor current is controlled to increase.
The present inventors conducted a test to investigate the relationship between product deposition and current changes in the motor M of the exhaust pump P by creating simulated conditions of product deposition in part S of the gas passage (see FIG. 9). The results confirmed that, as shown in FIG. 10, from a point of time when the thickness of the deposit inside the pump exceeds 50% of the gas passage cross-sectional area (see the diagram with a product deposition ratio of 50% in FIG. 9), the motor current increases drastically.
Accordingly, detection of such an amount of change in the motor current enables detection of a deposit inside the pump, or estimation of the deposit thickness to predict the time for maintenance of the exhaust pump.
Note, the thickness of the deposit in the pump with which the motor current starts to rise is not necessarily 50% in all machine types but varies depending on the design. Further, if, for example, the flow rate of the gas flowing through the exhaust pump P is low as in Operating Condition 1 (under which 800 sccm of gas B flows) of the exhaust pump P, motor current changes are small as shown in FIG. 10 and FIG. 11, because of which a significant determination of an increase in the motor current is not possible.
Moreover, the motor current changes also depending on individual pumps P, or the temperature in the pump, even if the flow rate of the gas is the same (see FIG. 12). Therefore, it is necessary to detect an increase (AI) in the motor current of at least 10% or more in order to significantly determine that there is an increase in the motor current.
However, in actuality, exhaust pumps are used in a variety of ways by end users, and the amount or flow rate of gas not only changes variously depending on each apparatus or formula, but also changes constantly during an operation with the same formula. Therefore, a precise detection of an amount of product deposition based only on a function of detecting that the motor current has exceeded a certain threshold is very difficult.
A method has been proposed, therefore, as a means for precisely detecting a state of product deposition, wherein, as shown in FIG. 13, a “health check mode” is provided, during which the type and flow rate of gas flowing through the exhaust pump are set constant.